Autoregulation of immunity by complementary idiotypes in mice immunized against the epitope phosphorylcholine (Pc) or the T15 idiotype of antibody specific for Pc. While a prior autogenous anti-idiotype response markedly suppresses the complementary idiotype response and vice versa, neither response is diminished when mice are immunized to have simultaneous complementary responses. The absence of regulation is not due to emergence of different idiotypes. Regulation of ongoing idiotypic response is not demonstrably dependent on complementary Ts cells. Nor is the absence of regulation occurring in simultaneous complementary responses due to mutual annihilation or neutralization of complementary Ts cells. Insofar as I can determine, Ig produced by one clone protects the clone from regulation by the Ig or cells of the complementary clone. Thus, the theme of the current work is to test the general concept that an essential function of Ig is to protect the clone that produces it.